I. Field of the Invention
This invention relates generally to the field of forming three-dimensional objects through a layer by layer buildup in accordance with the principles of stereolithography, and, more specifically, to increasing the surface resolution of such objects by the creation of thin fill layers to fill the surface discontinuities which inherently form in such objects in the course of building up the object layer by layer.
II. Background of the Invention
A variety of techniques for step wise building of three-dimensional objects have recently become available. One such technique is stereolithography, which is described in U.S. Pat. Nos. 4,575,330 and 4,929,402, the disclosures of which are hereby fully incorporated by reference herein (hereinafter the '330 and the '402 patents). In one embodiment of stereolithography, a three-dimensional object is formed in accordance with a corresponding object representation through a step wise laminar buildup of cross-sections of the object at the working surface of a polymerizable resin, which cross-sections are formed at the surface by selective exposure to UV radiation. Moreover, in accordance with this embodiment, the cross-sections, as they form, are adhered to previously-formed cross-sections through the natural adhesive properties of the polymerizable resin as it solidifies. FIG. 1 illustrates a side view of a spherical object 1 which is formed by the stepwise layer by layer buildup of stereolithography. The layers of the object are identified by numerals 1a, 1b, and 1c, respectively. Also shown is a corresponding object representation 2 which may be a data representation of the surface of the object originating from a CAD system (this representation may later be sliced into cross-sectional representations which are used to drive the stereolithography process). In FIG. 1, the object representation is depicted to be a representation of the surface of the object, and it appears as a circular envelope around formed object 1. The formed object 1 is depicted by the hatched area. Also shown are stair step surface discontinuities 3a through 3x which comprise deviations between the object 1 and the object representation 2. These surfaces discontinuities inherently form in objects produced through stereolithography, and result from the layers being used to form such objects having finite thicknesses. If, as will be discussed subsequently, infinitesimally small thin layers could be utilized, the surface discontinuities would be eliminated entirely. However, as will also be discussed subsequently, thin layers may, in general, not a feasible solution for reducing the surface discontinuities, and other techniques must be employed.
Additional details of stereolithography are available in the following co-pending patent applications, the disclosures of which, including appendices, are hereby fully incorporated by reference as though set forth in full herein:
______________________________________ U.S. APP. SERIAL FILING INVENT- NO. DATE OR(S) STATUS ______________________________________ 07/182,801 4/18/88 Hull, et al. U.S. PAT. NO. 4,999,143 07/183,015 4/18/88 Smalley U.S. PAT. NO. 5,015,424 07/268,428 11/8/88 Freed ABANDONED 07/268,429 11/8/88 Modrek et al. U.S. PAT. NO. 5,076,974 07/268,816 11/8/88 Spence U.S. PAT. NO. 5,058,988 07/268,837 11/8/88 Spence U.S. PAT. NO. 5,123,734 07/268,907 11/8/88 Spence U.S. PAT. NO. 5,059,021 07/331,644 3/31/89 Hull et al. ALLOWED 07/339,246 4/7/89 Hull et al. U.S. PAT NO. 5,104,592 07/365,444 6/12/89 Leyden ALLOWED 07/414,200 9/28/89 Hull et al. PENDING 07/415,168 9/29/89 Sekowski PENDING et al. 07/427,885 10/27/89 Spence et al. PENDING 07/429,911 10/27/89 Spence et al. U.S. PAT. NO. 5,133,987 07/428,492 10/27/89 Vorgitch ABANDONED et al. 07/429,435 10/30/89 Hull et al. U.S. PAT. NO. 5,130,064 07/429,301 10/30/89 Lewis et al. PENDING 07/495,791 3/19/90 Jacobs et al. PENDING 07/515,479 4/27/90 Almquist ALLOWED et al. 07/545,517 6/28/90 Cohen U.S. PAT. NO. 5,096,530 07/566,527 8/13/90 Jacobs et al. ABANDONED ______________________________________
Additional details of stereolithography are also available in two related applications which are being filed currently herewith. The disclosures of these additional applications are hereby fully incorporated by reference herein as though set forth in full.
The first of these concurrently-filed applications is U.S. patent application Ser. No. 07/606,191, entitled "Boolean Layer Comparison Slice," filed by Snead et al. This application discloses an apparatus and method for slicing an object representation into a plurality of cross-sectional representations, utilizing boolean operations to compare the boundaries of successive layers, which cross-sectional representations are subsequently used to drive a stereolithographic apparatus to produce successive object cross-sections in a layer by layer buildup of the object.
U.S. patent application Ser. No. 07/427,885, now U.S. Pat. No. 5,133,987, incorporated by reference above, describes various stereolithography apparatus design considerations and in particular describes design considerations related to the exposure system. The following three paragraphs have been extracted from this referenced application.
There are a number of considerations which are advantageously undertaken in the design of a stereolithographic apparatus. When using a dynamic mirror system to trace the desired pattern, a laser beam may be used which is passed through a converging lens before being directed by the mirrors to the working surface. This lens is chosen to bring the beam to a focus on the working surface of curable material which is often a liquid photocurable resin. Before passing through this converging lens, the beam may be passed through a diverging lens in order to increase its size and thereby allow a smaller image point to be formed after being focused by the converging lens. All parts of the horizontal liquid surface upon which the beam is to be traced do not have the same path length from the dynamic mirror system; and, therefore, the beam may not be in optimum focus at all such parts of the horizontal liquid surface. The beam must be focused to a relatively fine point so that maximum resolution of details may be realized in the part formed. Because of this focusing problem, a system employing a small field of view relative to the beam path length is desirable. This small field of view refers to the target surface dimensions (maximum width) being small relative to the path length between the scanning mirrors and the target surface. In other words, the angular displacement of the scanning mirrors should be small when traversing between extremes on the target surface. This design criteria is at odds with the need to make relatively large parts with such a system and keep the size of the system within reasonable limits.
The orientation of the beam is also of importance. As cure rates are affected by beam intensity (power/per unit area), it is advantageous to have a relatively uniform orientation of the beam on the surface. Similarly, a problem can occur whenever solidifying radiation impinges on the target surface at angles other than ninety degrees. When this happens, resin will be cured at these same angles, giving rigs to a roughness of part finish known as the shingle effect. Therefore, a design consideration is to have the beam as close to perpendicular to the liquid surface as possible. Again, small patterns relative to the length of beam path enhance this desired condition.
It should be noted that the shingle effect is reduced by building layers which are thin relative to the error which can be tolerated. This is because it is the displacement in the X-Y position of the beam at the liquid surface and the X-Y position of the beam at one layer thickness below the surface that gives rise to the error which causes the shingle effect. The thinner the layers, the more off perpendicular to the beam can strike the surface without producing significant shingling. In equation form, the maximum angular displacement of the beam from off center, .theta., is equal to the arctangent of the error which can be tolerated divided by the layer thickness. For an error tolerance of 2 mils and a layer thickness of 20 mils, for example, the maximum angular displacement of the beam can be about 5.7 degrees. However, if the layer thickness is reduced to 5 mils, the maximum angular displacement can be increased to about 21 degrees.
The second of these applications is U.S. patent application Ser. No. 07/606,802, entitled "Simultaneous Multiple Layer Curing for Forming Three-Dimensional Objects," filed by Smalley et al. This application describes methods of building high resolution objects from traditionally low resolution combinations of building materials and synergistic stimulation, which combinations result in a particularly deep (and therefore low resolution) cure depth. This is accomplished by delaying the exposure of certain areas on a particular cross-section until layers of material for additional cross-sections have been placed over the particular cross-section at which point, corresponding areas on the upper cross-sections are exposed. The number of these upper cross-sections are determined based on the cure depth being deep enough to penetrate and transform the specific areas at issue on the cross-section at hand. If exposure of these areas had not been delayed, because of the cure depth involved, resolution would have been negatively impacted, because material on lower cross-sections than the cross-section at hand would be inadvertently cured.
Turning to FIG. 2, the magnitude of the deviations and the corresponding discontinuities depend on the location of the discontinuity on the object surface. A "feature" of the object comprises a particular localized area on the surface of the object, and in FIG. 2, numeral 2a identifies a horizontal feature, while numeral 2b identifies a vertical feature. Therefore, as seen in the figure, the magnitude of the discontinuity is negligible at horizontal and vertical regions 2a and 2b, respectively. The only surface regions where the deviations are material are those regions which comprise neither completely horizontal nor nearly vertical features. Additionally, FIG. 1 depicts an undersized object while FIG. 2 depicts an oversized object.
Several techniques have been proposed to eliminate these surface discontinuities. As will be seen below, each one has one or more attendant problems, which prevent the technique from having universal applicability over a wide range of part geometries.
As mentioned earlier, a first proposed technique is the use of thinner layers. The technique of using thinner layers in the context of an undersized part is illustrated in FIGS. 4a and 4b. FIG. 4a is similar to FIG. 1 in that it depicts a side view of a spherical object and its corresponding object representation, but it differs from FIG. 1 in that the layer thickness of FIG. 4a, identified with numeral 4', is about half as much as that of FIG. 1, identified with numeral 4. As shown, the corresponding surface deviations 3a' through 3x' of FIG. 4a are smaller than the surface deviations 3a through 3x of FIG. 1. FIG. 4b and FIG. 2 are similarly related, in the context of an oversized part, except that the layer thickness in FIG. 4b, also identified with numeral 4', has about half the layer thickness of the formed object of FIG. 2, identified with numeral 4.
This technique, however, is not generally feasible with the typical combinations of building material and synergistic stimulation available. For example, typical building materials may not be able to form layers which are thin enough to adequately reduce surface discontinuities. Therefore, the individual layers once formed may not be adequately cohesive while in an unsupported state. This problem is more fully described in the concurrently filed U.S. patent application Ser. No. 07/606,802, entitled "Simultaneous Multiple Layer Curing for Forming Three-Dimensional Objects". Even if an appropriate material could be obtained, thin layers can be excessively affected by various forces that are applied to the part during its formation, thereby resulting in the material still being incapable of forming adequately cohesive layers. These forces include forces responsible for curl distortion, and those which are applied to the part during formation. For example, using an overdip recoating process, the layers of the part experience drag and gravitational forces as the partially-formed part is dipped up and down repeatedly in a vat of material in the course of successively coating top-most layers of the partially-formed part with layers of untransformed material to be utilized in the formation of successive part layers.
Another problem with thin layers is the longer layer formation time that may be required to form them. As discussed in the '330 patent, when using an overdip recoating technique, the partially-formed part is typically over-dipped into the vat of material so that excess material flows over the top-most layer. Then, the partially-formed part is typically up-dipped, and excess material is allowed to flow off or is swept away by means of a doctor blade or the like to form an untransformed layer of relatively uniform thickness. Because of the viscosity of the material involved, it may take an unacceptably long time for the excess material to flow off and leave a thin uniform layer of untransformed material. In addition, as described in Ser. No. 515,479, the use of a doctor blade to speed up this process may not be possible because of various problems attendant with the formation of thin layers.
A further problem with thin layers is that it may be difficult to control the exposure of the building material to the synergistic stimulation both to ensure that regions intended to be transformed are not exposed too much, resulting in a greater thickness than desired, and to ensure that regions not intended to be transformed are not inadvertently exposed since exposure may result in creating undesired solidified regions. In the case where the material is transformed upon exposure to a beam of radiation emitted by a source of synergistic stimulation, exposure control is typically performed by means of rotatable scanning mirrors which direct the beam to a working surface of building material, and by means of a shutter. These mirrors are described in the '330 patent, and in U.S. patent application Serial Nos. 07/331,644; 07/268,816; 07/268,907; 07/268,837; and 07/428,492. The shutter is described in Ser. No. 07/428,492. The specific control problems are obtaining mirrors which can be rotated fast enough both to guarantee the formation of thin enough layers, and also to prevent appreciable exposure over areas that are not to be transformed.
Another technique which has been proposed is an oversized building technique (known as STYLE 1 or 2) described in Ser. No. 07/331,664, according to which an oversized object is built which deviates from a corresponding object representation by means of solid deviations, which solid deviations are sanded down in a post-processing step. FIG. 2 illustrates an oversized object. Compared with FIG. 1, surface discontinuities 3a through 3x are shown which extend beyond the envelope of the object representation 2 thereby giving justification for the designation of STYLES 1 and 2 as oversized building styles. The extent of these deviations is shown by the cross-hatched areas in FIG. 2. These discontinuities or deviations could be sanded down so that the sanded object would more closely match the object representation.
A problem with this technique, however, is that it is impossible or extremely difficult to sand down surface deviations located in relatively inaccessible areas, since it may be difficult to situate an appropriate sanding apparatus in these areas. Therefore, these styles can only be used with a subset of possible part geometries where the deviations are situated in accessible areas.
Another disadvantage with this techniques is that it requires a manual post-processing sanding step, which can be labor-intensive, time-consuming, is largely incapable of being automated, and is therefore expensive.
A third technique which has been proposed is an undersized building technique (known as STYLE 3 or 4) described in U.S. patent application Ser. No. 07/331,644, according to which an undersized object is built which deviates from an object representation by means of hollow deviations. For this technique, the hollow deviations are filled in with material, and this material is then exposed to synergistic stimulation in a subsequent post-processing step. An undersized object is illustrated in FIG. 1, and the hollow surface deviations 3a through 3x are also illustrated. These deviations would be filled in with material and exposed in a subsequent post-processing step so that the resultant object would more closely match the object representation.
A benefit of this technique, compared to the oversized building technique, is that there are fewer inaccessible part geometries. However, this technique still suffers from the disadvantage that it requires a subsequent post-processing step which is manual, labor-intensive, is largely incapable of being automated, requires additional equipment, and is therefore expensive.
An additional disadvantage of this technique, compared to the oversized technique, is that certain undersized objects may not be structurally sound. This problem is illustrated in FIG. 3. FIG. 3a depicts a sideview of an object representation which is a side view of a sphere with a rectangular hole passing horizontally through it. FIG. 3b shows the object formed with a given layer thickness using an oversized building technique (STYLE 1 or 2). FIG. 3c shows the same object formed with that same layer thickness (as that used in FIG. 3b) using an undersized building technique (STYLE 3 or 4). As shown, not only does the object have the characteristic surface discontinuities, it is also structurally unsound. Layers 1b and 1c (comprised of sections, 1b', 1c', and 1c"), for example, are completely separated from each other by gaps 2e and 2f. The same thing is true for layers 1l and 1m.
To alleviate this problem, special supports could be used during the building process. These supports would support the separated layers, i.e., layers 1b to 1c and 1l to 1m in the Figure, until they could be adhered to the other layers, thereby ensuring structural soundness. However, even with these supports, the gaps must still be subsequently filled in with material and exposed. Compared to the filling in of surface discontinuities, it would be difficult to fill in the gaps between layers with any degree of accuracy. Therefore, application of specialized supports do not provide a general solution to the separation problem.
A fourth technique which has been proposed is described in Ser. No. 415,168. This application describes filling in the surface deviations simply by immersing the object in a liquid, or subjecting it to electrostatic powder coating, whereby the deviations are smoothed out by the action of surface tension. This technique requires additional equipment, and is therefore expensive. It additionally suffers from the same problems as does the undersized building style, and, additionally, for the accurate formation of parts, it requires the development of a slice program or the like which not only builds an undersized object as previously described but one which undersizes the object further to compensate for the coating thickness that will be formed even in surface areas where there are no surface discontinuities.
For all the foregoing reasons, an object of the present invention is to provide an apparatus for and method of reducing surface discontinuities in a three-dimensional object formed by stereolithography, which is general enough to apply to a wide range of object geometries. It is a further object to provide such an apparatus and method which can be implemented on the same apparatus used to build the object in the first instance, which does not require further equipment, and which is capable of being automated compared to the previously-described techniques. It is an even further object to provide such an apparatus and method which employs the use of thin layers to improve surface resolution, but which does not have the problems traditionally inherent in the use of thin layers, i.e., slow speed of layer formation, lack of structural strength, and the like.
Additional objects and advantages will be set forth in the description which follows or will be apparent to those of skill in the art who practice the invention.